1. Field of the Invention
This invention relates to an improved apparatus for flash-extruding a polymer solution to form a plexifilamentary strand. More particularly, it relates to an improvement in the nozzle of the spinneret pack disclosed in U.S. Pat. No. 3,484,899.
2. Description of the Prior Art
Apparatus for flash-extruding a polymer solution to form a plexifilamentary strand is disclosed by Blades and White, U.S. Pat. No. 3,081,519. Various flash-extrusion spinnerets are disclosed in the patent. For example, FIG. 9 of the patent illustrates a nozzle that contains a flared passage immediately downstream of and coaxial with the exit orifice. The flared passage has a flare angle, as hereinafter defined, of 90.degree. and an entrance diameter equal to the orifice diameter. Smith, U.S. Pat. No. 3,484,899 discloses a cylindrical passage of constant diameter, immediately downstream of and coaxial with the orifice. Passages of this type, having a constant diameter about five times as large as the diameter of the orifice have been used commercially. These downstream-of-the-orifice passages are often referred to as "tunnels".
The present inventor has found that after a flash-extrusion apparatus having a flared passage of the general type disclosed by Blades and White has been operated continuously for several hours, further operation produces flash-extruded strands that contain defects, which are referred to as "spits". A "spit" defect exhibits itself as an agglomerated group of fibrils which appear as a particle, sometimes as large in cross-section as an eraser on the end of a pencil, in an otherwise well-fibrillated plexifilamentary strand. When such strands are formed into nonwoven sheets, the "spit" appears as an obvious nonuniformity in the sheet.
The present inventor has also found that when attempting to increase the throughput through a flash-extrusion apparatus having a cylindrical tunnel of the general type disclosed by Smith, the plexifilamentary strands decrease in tenacity as throughput is increased.
The object of the improved apparatus of the present invention is to avoid or at least minimize the problems of spits and reduced tenacity associated with the above-described prior art apparatus.